It is a well known that chlorine-containing bleaching agents give rise to chlorinated aromatic substances. The major part can not be destroyed by biological purification of the sewage water. Some chlorinated compounds discharged with spent bleach liquor are bioaccumulatable and taken up by fish. Some chlorinated products have been found to be mutagens.
Consequently, disposal of chlorine-containing waste bleaching liquor from bleaching plants constitutes a very serious problem. Efforts have been made to reduce the use of free or elementary chlorine in the bleaching of cellulose pulp by use of chlorine dioxide instead. The production of chlorine dioxide requires about three times as much electrical energy per kilogram of active chlorine as elementary chlorine.
Nitrogen dioxide has been proposed as a substitute for chlorine in the bleaching delignification of cellulose pulp, and has been studied by Clarke (Paper Trade Journal, Tappi, Sect. 118 62 (1944)). Clarke has found that cellulose pulp can be partially delignified by treating the pulp in an aqueous suspension for from 1 to 1.5 hours at 90.degree. C. with nitrogen dioxide, followed by extraction at 90.degree. C. for 30 minutes, or at 50.degree. C. for 60 minutes at a 7% pulp consistency and an alkali charge corresponding to 2% NaOH, calculated on the dry weight of the pulp. The treatment results in a severe depolymerization of the cellulose, which is reflected in a very low viscosity of the treated pulp, compared with pulp subjected to chlorination and alkali extraction.
Bourit (French patent specification No. 2,158,873) avoids depolymerization by applying a delignification process in which the pulp is treated with nitrogen dioxide at low temperature, preferably a temperature below 20.degree. C., and for a long period of time, followed by an alkali extraction under mild conditions. The cellulose pulp is only delignified to a very small extent, however, and the method does not afford any solution to existing environmental problems.
The delignification of lignocellulosic material by treatment with nitrogen dioxide, followed by washing with water, treatment with alkali, and subsequent treatment with oxygen gas, has also been proposed in Swedish patent application No. 77 05136-5. However, this technique has not been put into commercial practice, because although enabling a high degree of delignification, the method causes a drastic lowering of the viscosity.
Another proposal which has not come into practice has been made in Swedish patent application No. 75 06646-4. This bleaching process includes the steps of (1) treating the cellulosic material with a blend of nitrogen monoxide and nitrogen dioxide with nitrogen monoxide in a molar excess, (2) washing with water, and (3) then treating with alkali, for example, in the presence of oxygen gas, under superatmospheric pressure. The nitrogen dioxide can optionally be generated in situ from nitrogen monoxide and oxygen, in which case the nitrogen monoxide is added in an excess of four times the added molar amount of oxygen. The reaction proceeds under superatmospheric pressure with respect to nitrogen monoxide; for example, 7 kp/cm.sup.2 is shown in Example 1. The nitrogen oxides are removed by depressurizing, followed by evacuation. In every Example, a superatmospheric pressure is employed in the handling of the nitrogen oxides. The handling problems remain, with a great risk of injury to both the internal and external surroundings, and a high consumption of nitrogen oxides. This method also results in a considerable lowering of the viscosity, although it does enable a high degree of delignification to be obtained.
When the pretreatment with nitrogen oxide is followed by an oxygen gas bleaching stage, it is said to be suitable, subsequent to displacing or washing from the pulp pulping liquor derived from a pulping process with the use of waste liquor derived from the oxygen gas bleaching, to wash the pulp with the acid washing liquid obtained in the washing stage after the pretreatment. When the acid washing liquid is not washed from the pulp before treating the pulp with nitrogen dioxide, the pH of the liquid is reported to be 2.0, which corresponds to about 0.01 gmole nitric acid, calculated per kg of water in the pulp. The prime object of the method is to remove harmful metal compounds from the pulp.
In summary, the pretreatment of cellulose pulp with nitrogen dioxide NO.sub.2 before an oxygen gas bleaching makes possible a more complete delignification and an improved oxygen gas bleaching, without deterioration in the paper-making properties of the pulp. However, relatively large quantities of nitrogen oxides and starting material (ammonia) for the manufacture of said oxides, respectively, are consumed in the process.
Brink U.S. Pat. No. 4,076,579, patented Feb. 28, 1978, delignifies particulate lignocellulosic material by nitric acid, which is formed in situ by first treating the lignocellulosic material with nitric oxide, and then reacting the nitric oxide with molecular oxygen. Brink intends to provide a higher-yield pulping process than current commercial alkaline pulping processes, one which can be conducted in an initial reaction at atmospheric pressure and relatively low temperature.
The pulping process is described in detail beginning at columns 3, line 20.
While Brink is concerned with a delignification requiring nitric acid, in contrast to an activation involving nitrogen dioxide and oxygen gas, Brink does form nitric acid in situ from nitrogen oxides that can react with moisture to form nitric acid, and preferably most advantageously, from nitric oxide NO, although nitrogen tetroxide (N.sub.2 O.sub.4), nitrogen trioxide (N.sub.2 O.sub.3), nitrogen dioxide (NO.sub.2) and nitrate ions, nitric ions, nitronium ions, and nitrosonium ions are also suggested.
Nowhere however does Brink suggest a reaction with nitrogen dioxide, NO.sub.2, and oxygen.
There is no teaching of a reaction between wood (or pulp) and NO.sub.2 /O.sub.2 ; there is only a teaching of a reaction between wood (or pulp) and HNO.sub.3.
When applied to lignocellulosic material, the Brink process involves a completely different set of reactions from an activation, applied to pulp. Brink's reactions with nitric acid also can involve different constituents of the lignocellulosic material (or pulp).
In addition to application to lignocellulosic material, under time and temperature conditions where represent pulping conditions, as a first stage, Brink also applies the process as a second stage, in treating the pulp product resulting from the first stage, using a lower temperature, and a higher moisture content of the lignocellulosic material; the reaction time is the same. Nonetheless, Brink does not disclose reaction of pulp with NO.sub.2 and O.sub.2. Neither does Brink disclose a combined activation by NO.sub.2 -O.sub.2 followed by oxygen gas bleaching.
In accordance with Samuelson, U.S. Ser. No. 270,438, filed June 4, 1981, the activation stage is carried out with nitrogen dioxide gas in the presence of oxygen gas in an amount such that nitrogen monoxide formed as an intermediate is consumed, while regulating the amount of oxygen gas that is added in such a manner that at the conculsion of the activation stage practically all of the nigrogen monoxide and nitrogen dioxide have been consumed.